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An engineer’s guide to selecting a drive

For use in potentially explosive environments, ATEX certification is an option
For use in potentially explosive environments, ATEX certification is an option
Credit: Deprag Schulz
Selecting a drive involves careful consideration when looking for the optimal motor for a given application. Dagmar Dàƒ¼bbelde looks at how to avoid design errors and save money

Right from the initial selection of a drive, a designer has to make careful considerations when looking for the optimal motor for their application. Determining the torque and speed, selecting a pneumatic motor from suppliers’ catalogues and clarifying the size and connections required for the machine – it sounds simple. But unfortunately it is not that easy, and a rude awakening may follow if, after installation, the selected pneumatic motor does not produce the power required. Determining the problem may take weeks; in the worst-case scenario the entire drive may have to be redesigned and purchased again. Therefore it is worthwhile to talk to the specialists at the beginning of the drive design phase.

Deprag Schulz has developed a simple guide with six steps to follow so that nothing is forgotten.

The selection of a pneumatic motor is not difficult. But, particularly with pneumatic systems, there are many factors which can decisively influence the power of an air motor. If, for example, when installing a motor it is found that too short a hose has been chosen, then this will drastically reduce the torque of the motor.

You can imagine that the developer does not necessarily have the length of hoses at the forefront of their mind when designing the machine; equally; the connectors between the filter units and oiler are not considered to be particularly relevant. But it is exactly these throttle points which ultimately determine whether the machine works correctly and if the motor provides the right torque.

Steps to selecting the right motor

So what are these steps for the selection of the right motor? First of all, the drive system best suited to the customer’s application must be chosen. Then the materials of which the external parts of the motor are composed are determined. After these initial decisions, the theoretically required motor power can be calculated and all performance-influencing factors can be taken into consideration.

The fourth step is the integration of the motor into the complete system of the machine. You have to decide how the motor will be connected to the machine and which gears are required. Perhaps a brake will be necessary in order to ensure safety of the system. Then the durability of the machine must be guaranteed, and finally the purchasing and running costs of the motor must be calculated and optimized.

Pneumatic motors are available in diverse design options. Their application and the intended operating time are of great importance when selecting the right basic principles. An air vane motor is suitable for regular running cycles. If you wish to run it non-stop then you must consider the wear on the vanes and the shorter maintenance intervals this requires. In comparison, gear motors and turbines are maintenance-free and therefore better suited to continuous operation. In this case the required speed must be considered. Turbines and gear motors rotate in upper speed ranges at approximately 140,000 rpm. Vane motors are available which rotate at very low speeds, e.g., 1 rpm. Oil-free operation is also an option for all three drive principles. A slight loss of power must be taken into account with oil-free operation of a vane motor.

Different materials

The second step examines the motor’s construction material. If operating in a dry surrounding atmosphere and in normal stationary production, an inexpensive air motor made from cast iron will be sufficient. Deprag offers a wide spectrum of low-priced Basic Line motors. For installation in robots and machines there are a variety of grinding motors, drilling motors and milling motors available which are distinguished by their low weight and compact size.

For use in the food industry, pneumatic motors must be able to withstand cleaning agents and steam. The Deprag Advanced Line motors with external parts made from stainless steel are additionally sealed and lubricated with food industry standard USDA-H1 grease.

Pneumatic drives can even be operated underwater. In this case it is essential to determine the water depth required. If the motors must be started underwater they can be used up to a depth of 5 metres. If the motors are started on the surface and then submerged, they can be used in a depth of up to 20 metres without damaging the motor. If the motor must be sterilizable, as requirements demand in some medical technology applications, then it can be equipped with special vanes. There are many examples here of why it is important to speak to the air motor manufacturers in advance about your application and to describe it in as much detail as possible.

Calculation of theoretical motor power
Calculation of theoretical motor power
Credit: Deprag Schulz

Motor power

The next step is the calculation of the theoretical motor power. Motors which are designed for use in only one rotational direction are more efficient than reversible motors. When determining the rotational direction, the pneumatic expert looks towards the motor shaft from the air inlet. This is the other way around for electric motors, where the rotational direction is specified by looking at the motor spindle.

First the required working point of the motor is determined: which nominal torque and speed under load do you want to reach? The most economic use of the motor (least wear and least air consumption) is attained by running close to the nominal speed. If you look at the characteristic curve of an air motor, it shows that it reaches its maximum torque just before standstill (around twice the specified nominal torque).

At the nominal speed the air motor reaches maximum power. With the formula nominal torque times working speed (nominal speed) divided by 9550 you can find the theoretically required power in kW.

In manufacturers’ catalogues, performance data is based on varied operating pressures. At Deprag this is 6 bar. If the application only has 5 bar directly at the motor then the motor loses 23 per cent of its power. If there is only 4 bar available then motor power is reduced by 45 per cent. A differing operating pressure is so decisive that it must be taken into account at the start of the design phase using the adjustment table, in order to avoid nasty surprises.

Next, the air supply volume – which is specified by the air consumption in the manufacturers’ details – must be ensured. Every reduction in the width of the opening, whether on the feed hose itself or due to connection parts, filters, oilers or the exhaust hose and silencer all have an effect on the air volume. Therefore Deprag recommends an exhaust air throttle to its customers to regulate their speed. Using a throttle on the supply air reduces the speed of the motor but, at the same time, the torque is reduced as well. Exhaust air throttles can reduce the speed without great loss of torque. The exhaust throttle means that customers can better utilize the wider working range which air motors provide.

The optimal lifespan and performance of an air motor is reached with lubricated running (1-2 drops of oil per 1 mà‚³ air consumption). Unlubricated operation can lead to a loss of power of around 10-20 per cent.

Design integration

If the right motor with the required power has been found, then the next step is to integrate it into the design. Deprag provides various spindle designs and individual fixing methods.

A complete solution is often better value than seeking a gear solution separately. Within the Deprag range there are numerous air motors with integrated planetary gears, spur gears and worms gears. If you require additional safety then a holding brake can be recommended. In the manufacturer’s programme you can also find brake motors. For use in potentially explosive environments, there are also options with the required ATEX certification. Integration is concluded with the technical verification of the maximum permissible axial and radial load on the drive spindle of the air motor.

Air motors are powerful, durable and robust. Adherence to the framework conditions determined during the design phase and compliance with the instruction manual will ensure the longest possible life of the drive. These conditions include adhering to the recommended air quality, lubricated operation, maintenance intervals, a maximum length of the feed hose of three metres and sufficient opening widths of the feed hose and connection parts.

Considering costs

Finally, the purchase price is the predominant consideration in the acquisition of a new drive system. But the designer should also remember running expenses and consider the operating costs and price for maintenance and servicing. When planning and selecting a new system, the question must be asked: how readily available are replacement parts and what are their prices? Maintenance and repair service quotations ensure that this is calculable. Deprag’s Basic Line air motors are particularly maintenance-friendly. The patented vane exchange system enables an air motor’s vanes to be quickly replaced in situ using a key and tweezers. Operating costs are determined by the air consumption. The right choice of motor sets the course for low running costs. The closer the motor runs to nominal speed (50 per cent of the idle speed), the more efficiently the air is used.

Deprag Schulz has been using compressed air as a working medium for many decades. Deprag’s standard programme offers a wide range of options and, from this modular system, individual drive solutions for the required application can be developed and produced at an attractive price-performance ratio. Around 85 per cent of the firm’s projects in the field of air motors are special solutions which have been quickly and simply realized from its standard programme.

Dagmar Dàƒ¼bbelde is Product Manager, Air Motors at Deprag Schulz

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